The present invention relates to frequency synthesis in a mobile radio communications station and more particularly to such a mobile station having switched dual frequency synthesizers for communications reception and signal strength monitoring, respectively.
In mobile radio telephone systems, mobile stations are assigned to different radio base stations at different times depending on the movements of the mobile station and transmission characteristics between the mobile station and various base stations. In order to select the most suitable base station to handle communications with a particular mobile station, signal strength monitoring is performed. Conventionally, a particular mobile station has been monitored not only by its assigned base station but also by neighboring base stations to determine the most suitable base station for handling communications with the mobile station. When it becomes evident that a base station other than the base station to which it is presently assigned is better suited to handle communications with the mobile station, responsibility for the mobile station is transferred to the better suited base station in a procedure known as hand-off.
Recently, it has been proposed that signal strength monitoring be performed at both ends of the radio communications link, i.e., at the mobile stations as well as the base stations, to facilitate hand-off. Such a technique is referred to as "mobile assisted hand-off" (MAHO).
Mobile-assisted hand-off is particularly useful in digital cellular mobile telephone systems using time-division multiplex access (TDMA), since rapid deterioration of speech quality as the mobile approaches the cell border requires hand-off to be handled very quickly. The mobile station therefore prepares for hand-off in advance by measuring the received signal strength of a set of other channels during the inactive period between the receiving time slots. These measurements are used to assist in the hand-off process.
Typically, base stations are provided with a dedicated signal strength receiver but mobile stations are not. Instead, a single receiver in the mobile stations must be shared between the communications reception function and the signal strength monitoring function. In TDMA systems, this has necessitated tuning to a forward voice communications channel during a reception time slot, thereafter tuning to the frequency of a signal the strength of which is to be monitored, and then re-tuning to the forward voice communications channel in time for the next reception time slot. Since the time between reception time slots is short (typically 20 ms) and the frequency deviation tolerance to maintain good reception is exacting, stringent requirements are placed on the frequency synthesizer of the radio receiver. Constructing a frequency-agile frequency synthesizer that is both fast-switching and highly accurate necessarily entails considerable expense.